Earthquake Site Effects, Seismic Hazard Analysis (SHA)

EARTHQUAKE SITE EFFECTS

The amplitude and the frequency content of the earthquake motion varies.

They depend on the soil cover underlying the point of observation and the topographical location.

The ground motion amplitude depend on the properties and configuration of the material located near the surface. The parameters that govern this behaviour are known as impedance and absorption.

The seismic waves at the same epicentral distance are higher on low density, low velocity soil than that of high density, high velocity rock.

The second parameter, absorption, counteracts the increase in amplitude. Absorption is the damping associated with the propagation media and it is higher in soils compared to rocks.

When an elastic wave travels through a layered media, at the interface, part of the wave is transmitted and part of it gets reflected. This process is governed by the term, impedance ratio, a₂

Impedance ratio is defined as

where p₁ and V₁ are the density and wave velocity of the bottom layer and p₂ and  V₂ are that of top layer.

The amplitude of transmitted waves in case of one dimensional wave propagation given by.

Where A_i and A_t are the amplitudes of incident and transmitted waves.

From these, it can be observed that for a value of impedance ratio equal to zero, i.e., free surface, the amplitude of transmitted wave will be twice that of incident wave.

Similarly, an impendance ratio of 0.25 implies that transmitted wave will have 60% more amplitude compared with incident wave.

Effect of site topography

The location of the site in aridge or a valley will have profound effects on the incident motion.

In case of ridge that is approximated as a triangular wedge, the apex displacement is amplified by a factor of 2 π/Ø, where Ø is the vertex angle of the wedge.

Basin Effect

The curvature of the basin where the soft soil is deposited can trap the waves and amplify the motion experienced on the surface. This could also significantly increase the duration of the earthquake motion.

Seismic Hazard Analysis (SHA)

Seismic Hazard

Definition

It is defined as any physical phenomenon such as ground shaking or ground failure that is formed by an earthquake. It may produce adverse effects on human activities.

Seismic hazard analysis involves the quantitative estimation of ground shaking hazards at a particular area.

The most important factors affecting seismic hazard at a location are:

(i) Earthquake magnitude

(ii) Source-to-site distance

(iii) Earthquake rate of occurrence (return period)

 (iv) Duration of ground shaking

Types of Seismic Hazard Analysis

Seismic hazard is analyzed in two ways viz., such as,

(i) Deterministically (as when a particular earthquake scenario is assumed),

(ii) Probabilistically, (in which uncertainties earthquake size, location, tion, and time of of occurrence are A explicitly considered).